This invention relates to a hydraulic turbine, which may be optionally a pump turbine, and a stay ring to be used in a hydraulic turbine.
A hydraulic turbine generally has a penstock, a spiral casing, a stay ring, a set of guide vanes, a runner, a draft tube and a generator. When the hydraulic turbine is in operation, water from an upper reservoir is guided by the penstock and its flow direction is shifted to a rotating direction by the spiral casing. Then, the water is guided to the runner by way of the stay ring that links the spiral casing and the guide vanes for controlling the flow rate. Thus, the potential energy of the water is converted into rotational energy before the water flows out to a lower reservoir by way of the draft tube. The rotational energy collected by the runner is then converted into electric power by the generator.
The stay ring is formed by ring-shaped upper and lower walls and stay vanes that link them. The stay vanes take the role of rectifying the flow of water from the spiral casing, and, at the same time, the role of strengthening members linking the upper and lower walls.
The profiles of stay rings that can be used in hydraulic turbines are largely classified into two types. One type is a bell-mouthed type where the height of the stay ring differs between the spiral casing side and the guide vane side, and the other is a parallel type where it does not differ. Conventionally, stay rings of the bell-mouthed type are popular, because the wall surface of the spiral casing and the wall surfaces of the upper and lower walls of the stay ring of that type are linked smoothly. As for a stay ring of the parallel type, there exists a part that runs in parallel with the outer peripheries of the stay vanes. Thus, the flow direction of water flowing from the spiral casing into the stay ring varies as a function of the peripheral position, but the flow angle of water is substantially the same in the elevating direction at a same peripheral position. Therefore, the loss of energy is small if the stay vanes have an inlet profile having an angle that is substantially the same as the flow angle.
However, a large number of hydraulic turbines, which were built decades ago and have bell-mouthed type stay rings, are still operating now. Many of the hydraulic turbines that were built decades ago have stay rings whose profiles are not appropriate, consequently giving rise to large energy loss at the stay rings. Since the stay ring of the bell-mouthed type has upper and lower wall inclined surfaces, water flows aslant relative to the central axis of the hydraulic turbine near the upper and lower wall surfaces. Therefore, the flow angles differ between the flow angle near the upper and lower wall surfaces and the flow angle near the center. Thus, if the stay vanes have a same profile in the elevating direction, there exists a part in each of the stay vanes where the flow angle of water flowing into the stay vane and the geometrical angle of the inlet of the stay vane vary from each other, which can give rise to energy loss at the inlet part. Stay vanes to be used in a stay ring of a hydraulic machine that have varying inlet profiles in the elevating direction are known in the art (See Japanese Patent Application Laid-Open Publication No. 2000-297735, the entire content of which being incorporated herein by reference).
As pointed out above, the stay vanes take the role of strengthening members. This means that, when existing stay vanes are to be modified, the strength thereof needs to be taken into consideration. In short, existing stay vanes should not be modified to a large extent particularly if they are to be renovated by cutting. However, if the rate at which the profile of the upper and lower wall surfaces of a stay ring of the bell-mouthed type changes is not appropriate, the flow angle can show significant variances in the elevating direction at the inlet parts of the stay vanes. Then, it may be difficult to make the modification of the stay vanes sufficiently effective, if the modification is made only at the inlet parts thereof.